Peristaltic pumps are used in numerous applications that require low shear pumping, portability, ability to run dry, ease of cleaning, accurate dosing, etc. These applications can be found in industries ranging from pharmaceutical manufacturing to food processing to water treatment.
The basic principle of peristaltic pumping involves the rotation of a central rotor containing either rollers or fixed shoes against a resilient elastomeric tube surrounding the rotor that is compliant enough to allow for complete collapse from the rotating rollers, and yet elastic enough to recover to a circular cross-section (referred to as restitution) once the rollers pass, thus enabling the next segment of tubing to fill with the process fluid and maintain flow. Thus, the tubing must withstand repeated flexure in contact with the process fluid.
There are two main types of peristaltic pumps: tubing pumps and hose pumps. Tubing pumps typically contain rollers to compress small diameter tubes ranging in size from 0.5 m to 25 mm inside diameter. Tubing pumps are manufactured by several companies including Watson-Marlow Bredel, Ltd. (Falmouth, England), Ismatec SA (Glattbrugg, Switzerland), and the Barnant Company (Barrington, Ill.). Hose pumps typically contain fixed shoes attached to the rotor which are used to compress large diameter hoses that may contain reinforcing cords in the side wall and range in size from 10 mm to 100 mm in inside diameter. Hose pumps are manufactured by several companies including Bredel Hose Pumps BV (Delden, The Netherlands), Verder Deutschland GmbH (Haan, Germany), and Allweiler AG (Radolfzell, Germany).
One unique capability of peristaltic pumps is that shear sensitive products can be conveyed with either little or no damage to the product. For example, live fish and whole fruit have been pumped without degradation. In general, fluids containing suspended material, either fine or coarse, can be readily processed with peristaltic pumps. Centrifugal pumps, on the other hand, often have problems with damaging both the process product and the internal workings of the pump. Peristaltic pumps can also be run dry without the concern of destroying the pump. Other pump types, such as progressive cavity pumps and centrifugal pumps, are quickly damaged by operating without a fluid in the pumping chamber since they rely on the process fluid for lubrication.
Another advantage of peristaltic pumps is their relatively simple method of operation. This feature means that peristaltic pumps can be easily cleaned with the removal of the flexible tubing which is the only portion of the pump containing the process fluid. Once the tube is removed, the pump is ready for service with a different material. Centrifugal pumps, on the other hand, are difficult to clean completely due to the many crevices in the pumping chamber. In the case of air operated diaphragm pumps, the pump must be disassembled, have the diaphragms removed, and cleaned throughout the internal chamber in order to reduce cross-contamination. The cleaning costs associated with centrifugal, air operated diaphragm, and progressive cavity pumps are significant and lead to considerable down-time.
Another advantage of peristaltic pumps is that they can readily accept a wide range of tubing materials for various applications with non-aggressive fluids. Tubing materials commonly used in peristaltic pumping include silicone rubber, polyvinyl chloride (PVC) sold under the trademark of Tygon by Saint-Gobain Performance Plastics, Inc. (Akron, Ohio), ethylene-propylene-diene monomer rubber blended with polypropylene sold under the trademark of Marprene by Watson-Marlow Bredel, Ltd. and by Advanced Elastomer Systems, L.P. (Akron, Ohio) under the trademark of Santoprene, polyisoprene, natural rubber, polychloroprene, polyurethanes, and blends of elastomers. Thus, for example, applications requiring long life and low operating cost may choose a thermoplastic elastomer tubing. Applications requiring high purity and stable flow rates may choose silicone tubing. As a result, the end user can accommodate the process fluid by judiciously selecting the proper tubing material that is compatible with their particular fluid.
Unlike tubing, hose construction typically involves a layer of pure elastomer such as natural rubber, covered by layers of either tire cords or reinforcing yarns, and covered further by a layer of abrasion resistant butadiene mixed with natural rubber, as described by Boast (EP 325 470 B1). The reinforcing filaments in hoses allow hose pumps to operate at higher back pressures compared to tubing pumps.
Although peristaltic pumps have many advantages, they do suffer from some drawbacks. In particular, pump tube materials are typically not compatible with aggressive chemicals. Process streams containing solvents tend to extract plasticizers used in thermoplastic tubing, such as polyvinyl chloride. Solvents can severely swell thermoset elastomers, such as silicone rubber and natural rubber. Other chemicals result in chemical degradation of the polymeric tubing. As a result, the application of peristaltic pumps in numerous industries has been limited. Applications such as metering strong acids and bases, transferring solvent laden waste streams, transferring agrochemical compounds, dispensing printing inks, metering reactors with active pharmaceutical intermediates, and the recovery of hazardous materials have all been hampered without the availability of a chemical resistant tube and hose.
Fluoropolymers are known for their excellent chemical resistance. Fitter (U.S. Pat No. 3,875,970) described a polytetrafluoroethylene (PTFE) lined silicone rubber tube. Although not shown by example, the inventor claims that this combination should provide improved resistance to chemical attack. PTFE possesses excellent chemical resistance; however, it exhibits poor flexure endurance when it has not been stretched and expanded into a highly oriented structure as demonstrated by the instant invention.
Gore (U.S. Pat. No. 3,953,566) teaches a method of stretching and expanding PTFE to orient the polymer, thereby improving its mechanical properties. The “expanded” PTFE film results in a node and fibril morphology with a high degree of orientation. The porous PTFE is useful in many applications requiring breathability, strength, and flex endurance; however, it is not suitable for containing process fluids due to its porosity.
Knox (U.S. Pat. No. 5,374,473) describes the preparation of a full density expanded PTFE film for fluid handling applications such as pump diaphragms; however, the method of fabrication requires heating the expanded PTFE membranes to 368° C. for 55 min. in a high pressure autoclave (17 atm.) while evacuating the PTFE film encapsulated within a polyimide vacuum bag and breather cloth in order to render the film substantially non-porous. This process is not economically viable for the production of peristaltic pump tube liners due to the cost of the disposable vacuum bags and the operation of the autoclave.
Sunden (U.S. Pat. No. 5,482,447) taught the use of a rigid fluoroplastic tube contained within another rigid fluoroplastic tube such that the outside diameter of the inner tube was close to the inside diameter of the outer tube. The inside diameter of the inner tube was claimed to have a range of 0.5 to 18 mm. Commercially available tubes from Barnant Company are limited to 4 mm in inside diameter, thus restricting the range of achievable flow rates. Those skilled in the art recognize that larger bore to wall ratio tubes have difficulty restituting without the aid of an elastomeric covering due to the plastic deformation and creep inherent in thermoplastic fluoropolymers.
As a result, there is considerable need for a fluoroplastic lined elastomeric pump tube that has significant usable flex life to pump aggressive chemicals and does not suffer from the creep and lack of resilience observed in pure fluoroplastic tubes. There is also a need for much larger diameter fluoroplastic liners for peristaltic hose pumping. There is a further need for flexible elements for pinch valves. There is also a need for flex endurant elastomeric diaphragms.